Quantum-enhanced coherent Raman spectroscopy with broadband squeezed-light detection.

Quantum correlations offer the possibility of surpassing classical limits in optical measurements, but their integration into nonlinear vibrational spectroscopy has remained largely unexplored. Here, we introduce a quantum-enhanced implementation of coherent Raman spectroscopy that employs broadband squeezed-light detection to interrogate relative intensity correlations between Stokes and anti-Stokes fields generated in an ultrafast four-wave-mixing process. By exploiting nonclassical correlations between these fields, we suppress the nonresonant background and enhance the signal-to-noise performance in both temporal and spectral domains, enabling detection sensitivities beyond the classical shot-noise limit. Importantly, the quantum advantage demonstrated here does not arise solely from improved detection statistics. Rather, it originates from microscopic intermolecular correlations mediated by cascaded exchange of a virtual photon between molecular pairs. This mechanism establishes a fundamentally distinct and significant pathway for information extraction in vibrational spectroscopy, providing access to structural and dynamical observables that are intrinsically inaccessible by conventional classical Raman techniques. Numerical simulations illustrate the power of the technique by resolving intradimer [Formula: see text] bond splitting in cyclobutane thymine dimers following ultraviolet excitation, highlighting its potential for probing ultrafast photochemical processes with quantum-limited sensitivity and for label-free biomedical optical sensing and imaging.